I've written the following code:
#include <iostream>
inline namespace M
{
int j=42;
}
int main(){ std::cout << j << "\n"; } //j is unqualified name here.
//Hence, unqualified name lookup rules will be applied.
//This implies that member of inline namespace shall not be considered.
//But it is not true
And it works fine. But I'm expected that the that program is ill-formed. It is because the Standard said (N3797, sec. 7.3.1/7):
Finally, looking up a name in the enclosing namespace via explicit
qualification (3.4.3.2) will include members of the inline namespace
brought in by the using-directive even if there are declarations of
that name in the enclosing namespace.
Also the section 3.4.1/6 does not said anything about involving of inline namespace in the unqualified name lookup:
A name used in the definition of a function following the function’s
declarator-id 28 that is a member of namespace N (where, only for the
purpose of exposition, N could represent the global scope) shall be
declared before its use in the block in which it is used or in one of
its enclosing blocks (6.3) or, shall be declared before its use in
namespace N or, if N is a nested namespace, shall be declared before
its use in one of N’s enclosing namespaces.
It is a g++ bug or I understood that rules incorrectly?
There's no bug..
No, it's not a bug in neither g++ (or clang++) which has the behavior described, the compiler is supposed to find j.
inline namespace N {
int j;
}
int main () {
int a = j; // legal, `j` == `N::j`
}
What does the Standard say?
You are missing a very important section of the standard, namely 7.3.1§8, where it states that the enclosing namespace of an inline namespace implicitly has a using directive that refers to the inline namespace.
[7.3.1]p8 namespace definition [namespace.def]
Members of an inline namespace can be used in most respects as thought they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using-directive (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1).
Elaboration
This means that our previous example is semantically equivalent to the below, where we have introduced a using-directive to bring the names from our nested namespace into the global namespace:
inline namespace N {
int j;
}
using namespace N; // the implicit using-directive
int main () {
int a = j; // legal
}
Related
Can someone explain why I can't define variable that was declared in anonymous namespace as global variable in another place?
#include <iostream>
namespace {
extern int number;
}
int number = 123;
void g() {
std::cout << number;
}
Compiler says that "Reference to 'number' is ambiguous" but I can't understand why it recognises declaration and definition as different things? Thank you in advance.
In general, the name being declared by a declaration is not looked up—after all, you can’t rely on finding a previous declaration when a name is first introduced. Of course, there is a similar process that traps things like
int x;
float x;
However, since it isn’t lookup it is not affected by using at all (including for an unnamed namespace). Another way of describing this distinction is that a declaration puts entities into namespaces and thus need not consider any other namespace in order to decide where to put an entity.
There are also cases where lookup does occur for (what might be) a declarator-id:
namespace N {using X=int;}
// using namespace N;
struct A {
A(X()); // ?
};
A has a member function with no parameters returning an A named X (with meaningless parentheses around its declarator); however, with the using-directive it instead has a constructor that takes a pointer to a function of no parameters returning an int. Similarly, in a declaration beginning
template<>
struct X<…
X must be fully looked up, even though a declaration of an explicit specialization must inhabit the same scope as the primary template (with leeway for inline namespaces), because it might continue
template<>
struct X<int>::Y<char> {…};
and not be a specialization of X at all.
For the unqualified name-lookup the compiler considers also nested unnamed namespaces in the global namespace,
You declared two different objects with the same name in the global namespace and in the nested unnamed namespace.
The using directive for unnamed namespace is implicitly inserted in the enclosing namespace.
Consider the following demonstration program
#include <iostream>
namespace N
{
extern int number;
}
using namespace N;
int number = 123;
int main()
{
std::cout << number << '\n';
}
The compiler will issue an error due to the ambiguity for the unqualified reference to the name number in this statement
std::cout << number << '\n';
The similar situation takes place with an unnamed namespace because the using directive is implicitly inserted in the enclosing namespace.
From the C++ 20 Standard (9.8.2 Namespace definition)
7 Members of an inline namespace can be used in most respects as
though they were members of the enclosing namespace. Specifically, the
inline namespace and its enclosing namespace are both added to the set
of associated namespaces used in argument-dependent lookup (6.5.3)
whenever one of them is, and a using directive (9.8.4) that names
the inline namespace is implicitly inserted into the enclosing
namespace as for an unnamed namespace (9.8.2.2).
There are two types of name hiding in c++:
1) Normal name hiding: [basic.scope.hiding]p1 (http://eel.is/c++draft/basic.scope.hiding#1):
A name can be hidden by an explicit declaration of that same name in a
nested declarative region or derived class ([class.member.lookup]).
2) The special type of name hiding in [basic.scope.hiding]p2 (http://eel.is/c++draft/basic.scope.hiding#2):
A class name ([class.name]) or enumeration name ([dcl.enum]) can be
hidden by the name of a variable, data member, function, or enumerator
declared in the same scope. If a class or enumeration name and a
variable, data member, function, or enumerator are declared in the
same scope (in any order) with the same name, the class or enumeration
name is hidden wherever the variable, data member, function, or
enumerator name is visible.
I'm interested to know about how name hiding interacts with using-directives when unqualified name lookup is performed.
For the first type of name hiding the behaviour is quite clear. This is because [basic.scope.hiding]p1 has been reformulated in terms of the rules in the section [basic.lookup.unqual] (http://eel.is/c++draft/basic.lookup.unqual)
The same has not been done for the second type of name hiding. So the following question now arises:
*) How should this second type of name hiding interact with unqualified name lookup that involves using-directives?
Looking elsewhere in the standard I find [namespace.udir]p2 (http://eel.is/c++draft/namespace.udir#2) and I think this is key to answering this question:
A using-directive specifies that the names in the nominated namespace
can be used in the scope in which the using-directive appears after
the using-directive. During unqualified name lookup
([basic.lookup.unqual]), the names appear as if they were declared in
the nearest enclosing namespace which contains both the
using-directive and the nominated namespace. [ Note: In this context,
“contains” means “contains directly or indirectly”. — end note ]
Applying the as if part of this rule to [basic.scope.hiding]p1 gives consistency with the rules in the section [basic.lookup.unqual]. This application is also consistent with [basic.scope.hiding]p4 (http://eel.is/c++draft/basic.scope.hiding#4) So this looks promising.
Because of this I think we can answer the question *) by similarly applying the as if part of [namespace.udir]p2 to [basic.scope.hiding]p2. This application is also consistent with [basic.scope.hiding]p4. I think this is also the most natural and least complex interpretation of the c++ standard.
The problem however is that Clang and GCC does not make the same interpretation as me. For example:
namespace N { static int i = 1; }
namespace M { struct i {}; }
using namespace M;
using namespace N;
int main() { sizeof(i); }
According to my interpretation this program should be well-formed and i should be looked up as the integer variable. Both Clang and GCC disagree with this by giving a name lookup ambiguity.
In the case of Clang this more complex interpretation leads to the following bug:
namespace N { static int i = 1; }
namespace M { struct i {}; }
namespace P {
using N::i;
using M::i;
}
namespace Q { using M::i; }
using namespace P;
using namespace Q;
int main() { sizeof (i); }
Gives no errors, but change
using namespace P;
using namespace Q;
into
using namespace Q;
using namespace P;
and we get name-lookup ambiguity error. GCC is at least consistent here.
Did I interpret the c++ standard correctly?
The key phrases here I believe are:
A name can be hidden by an explicit declaration of that same name in a nested declarative region or derived
class (10.2).
A class name (9.1) or enumeration name (7.2) can be hidden by the name of a variable, data member,
function, or enumerator declared in the same scope.
In this example:
namespace N { static int i = 1; }
namespace M { struct i {}; }
using namespace M;
using namespace N;
int main() { sizeof(i); }
Both is are declared in different, non-nested scopes, so there is no hiding. Name lookup finds them as if they were declared in ::, but that's not what the rule for hiding stipulates.
Otherwise, we have, from [basic.lookup]:
Name lookup shall
find an unambiguous declaration for the name (see 10.2). Name lookup may associate more than one declaration
with a name if it finds the name to be a function name;
There is no unambiguous declaration in ::, so this code is ill-formed and the error is correct. The same is true for the other example, so the fact that there is some using-declaration ordering for which clang compiles it is a bug.
While this is non-normative, there is an example in [namespace.udir] that makes this interpretation clear:
[ Note: In particular,
the name of a variable, function or enumerator does not hide the name of a class or enumeration declared
in a different namespace. For example,
namespace A {
class X { };
extern "C" int g();
extern "C++" int h();
}
namespace B {
void X(int);
extern "C" int g();
extern "C++" int h(int);
}
using namespace A;
using namespace B;
void f() {
X(1); // error: name X found in two namespaces
g(); // OK: name g refers to the same entity
h(); // OK: overload resolution selects A::h
}
—end note ]
namespace N { static int i = 1; }
namespace M { struct i {}; }
using namespace M;
using namespace N;
int main() { sizeof(i); }
This is ill-formed. §7.3.4/6:
If name lookup finds a declaration for a name in two different
namespaces, and the declarations do not declare the same entity and do
not declare functions, the use of the name is ill-formed.
The same applies to your second example. Note that the name hiding rule that applies in e.g.
struct A {} A;
…doesn't apply in your case, as the two is are declared in distinct scopes. Also,
During unqualified name lookup ([basic.lookup.unqual]), the names
appear as if they were declared in the nearest enclosing namespace
which contains both the using-directive and the nominated namespace.
Is irrelevant as well since any ambiguity that name lookup produces, as in your examples with i, is dealt with after lookup - here in e.g. the aforementioned §7.3.4/6.
Consider the following two code snippets:
Snippet A
#include <iostream>
namespace
{
bool foo = false;
}
bool foo = true;
int main()
{
std::cout << foo << std::endl;
}
Snippet B
#include <iostream>
namespace A
{
namespace
{
bool foo = false;
}
bool foo = true;
}
int main()
{
std::cout << A::foo << std::endl;
}
In Snippet A, foo's usage inside int main() is ambiguous, whilst in Snippet B it is not. Why is this the case?
Related: Anonymous Namespace Ambiguity
The behavior of unnamed namespaces is specified in §7.3.1.1 [namespace.unnamed]/p1:
An unnamed-namespace-definition behaves as if it were replaced by
inline_opt namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }
where inline appears if and only if it appears in the
unnamed-namespace-definition, all occurrences of unique in a translation unit are replaced by the same identifier, and this
identifier differs from all other identifiers in the entire program.
In particular, note that the declarations inside the unnamed namespace is made visible in the surrounding scope via a using-directive using namespace unique;.
In Snippet A, foo is unqualified, so the compiler performs unqualified name lookup (§3.4.1 [basic.lookup.unqual]). Relevant here is paragraph 2 of the subclause:
2 The declarations from the namespace nominated by a using-directive
become visible in a namespace enclosing the using-directive; see
7.3.4. For the purpose of the unqualified name lookup rules described in 3.4.1, the declarations from the namespace nominated by the
using-directive are considered members of that enclosing namespace.
Hence, unqualified name lookup finds both declarations of foo, and the name is ambiguous.
In Snippet B, A::foo is qualified, so qualified name lookup rules apply. Since A is a namespace, the applicable subclause is §3.4.3.2 [namespace.qual]. As relevant here, the rule is specified in paragraph 2 of that subclause:
For a namespace X and name m, the namespace-qualified lookup set
S(X,m) is defined as follows: Let S'(X,m) be the set of all
declarations of m in X and the inline namespace set of X
(7.3.1). If S'(X,m) is not empty, S(X,m) is S'(X,m); otherwise,
S(X,m) is the union of S(Ni,m) for all namespaces Ni nominated
by using-directives in X and its inline namespace set.
In other words, qualified name lookup considers namespaces nominated by using-directives only if the name is not found in the specified namespace and its inline namespace set. Here, the name foo is found in namespace A, so the unnamed namespace nominated by the using-directive is not considered, and there's no ambiguity.
If you write ::foo instead of foo in Snippet A, then qualified lookup rules would apply instead, and there would once again be no ambiguity.
Assume the following code:
namespace test
{
namespace detail
{
}
inline namespace v1
{
namespace detail
{
void foo()
{
}
}
}
}
int main()
{
test::detail::foo();
}
As we can see, this code compiles with Clang; not with GCC, however - GCC complains that the reference to namespace detail is ambiguous:
main.cpp:20:11: error: reference to 'detail' is ambiguous
test::detail::foo();
^
main.cpp:4:5: note: candidates are: namespace test::detail { }
{
^
main.cpp:10:9: note: namespace test::v1::detail { }
{
^
Which compiler does the correct thing here?
GCC is correct:
Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using-directive that names the namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1). Furthermore, each member of the inline namespace can subsequently be explicitly instantiated (14.7.2) or explicitly specialized (14.7.3) as though it were a member of the enclosing namespace. Finally, looking up a name in the enclosing namespace via explicit qualification (3.4.3.2) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.
(This is at 7.3.1/8 in old n3337 numbering)
I believe you're seeing Clang bug #10361.
GCC is correct.
N3797 states that,
and a using- directive (
7.3.4 ) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (
7.3.1.1 ).
Thus, test::detail is not the same namespace as test::v1::detail, so the lookup of test::detail is ambiguous. The Standard is exceptionally clear that the lookup of test::detail should include test::v1::detail, there are many quotes in this section to support this, but nothing to state that they should be considered the same namespace.
Arguably, I would say that Clang's behaviour is superior, but GCC's is correct.
There is a quote from 7.3.1/8 of N3797:
Members of an inline namespace can be used in most respects as though
they were members of the enclosing namespace.
Consider the following code snippet:
namespace M
{
int j = 7;
inline namespace MM
{
int j = 8;
}
}
I think that the example violates the ODR. But it is not true and it is compiling successful. Can you explain that behavior?
Introduction
7.3p1 Namespaces [basic.namespace]
A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.
A declared entity inside a namespace belongs to that namespace, ie. it's a member of that specific namespace, no matter if the namespace is inline or not.
ODR VIOLATION = N0NE
Your example snippet does not violate the ODR, mainly because you have 2 different entities named j;
namespace N {
int j = 0; // 1st
inline namespace M {
int j = 1; // 2nd
}
}
As pointed out further down in [namespace.def]p8, name lookup in the enclosing namespace will include those found in any inline namespace, but the members of the nested inline namespace are still entities of their own.
7.3.1p8 Namespace definition [namespace.def]
Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lokoup (3.4.2) whenever one of them is, and a using-direction (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1).
Furthermore, each member of the inline namespace can subsequently be explicitly instantiated (14.7.2) or explicitly specialized (14.7.3) as though it were a member of the enclosing namespace. Finally, lookup up a name in the enclosing namespace via explicit qualification (3.4.3.2) will include members of the inline namespace brought in by the using-directive even if there are declarations of the name in the enclosing namespace.
The added names are not treated as redeclarations of previously declared entities, they are additional names, in a nested declarative region, that are brought into the enclosing namespace during name-lookup.
Note: Relying on the compiler to issue a diagnostic in terms of ODR-violations is not safe, mainly because the Standard explicitly states that "no diagnostic [is] required" if an application violates the rules set up by [basic.def.odr].Further details are avaiable in a comment by Matthieu M. on this post.